The present invention relates to electric meters and, more particularly, to display temperature compensators for electronic demand registers of electric meters.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed which is proportional to the electric power being consumed by a load. Geared dials, or cyclometer discs, integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
In addition to the above measurement of consumption, some electric meters contain means for separating the consumption into those parts of consumption occurring during peak and off-peak hours (however defined) and for recording maximum demand in any of a contiguous set of demand intervals during a predetermined period of time in order to adjust billing according to such parameters. In one such demand meter disclosed in U.S. Pat. No. 3,586,974, a mechanical demand register records the power usage during each demand interval over the predetermined period of time and stores the maximum value for reading. The predetermined period of time may be, for example, the time between meter readings, or a period of time corresponding to the billing period of the utility providing the power. A clockwork mechanism restarts the demand register at the ends of regular demand intervals of, for example, a fraction of an hour, so that, at the end of the predetermined period, the stored value represents the highest value of power usage occurring during any one of the regular demand intervals in the predetermined period.
Demand registers of the mechanical type, such as disclosed in the above U.S. Patent, have limited flexibility. Once their design is completed for a particular meter physical configuration, the design is not transferrable to a meter having a different physical configuration. In addition, the demand-measurement functions cannot be redefined without major mechanical redesign.
Greater flexibility may be obtainable using electronic acquisition, integration and processing of power usage. An electronic processor such as, for example, a microprocessor may be employed to manage the acquisition, storage, processing and display of the usage and demand data. U.S. Pat. Nos. 4,179,654; 4,197,582; 4,229,795; 4,283,772; 4,301,508; 4,361,872 and 4,368,519, among others, illustrate the flexibility that electronic processing brings to the power and energy usage measurement. Each of these electronic measurement devices includes means for producing an electronic signal having a characteristic such as, for example, a frequency or a pulse repetition rate, which is related to the rate of power usage. The electronic processor is substituted for the mechanical demand register of the prior art to keep track of the power usage during defined periods of time.
Electronic processors require some compatible means for displaying data to the energy consumer and/or to the meter reader. If the electronic processor is a digital processor, it is preferable to employ a display which is compatible with such digital processor. One such compatible display is a liquid crystal display (hereinafter LCD) in which a voltage impressed in a pattern across a liquid crystal fluid creates a corresponding pattern of transparent and opaque area which may be used to visually convey data.
One type of liquid crystal display employs multiplexed drive of a plurality of identical symbols in order to drastically reduce the number of external connections required. Multiplexed drive is accomplished using a plurality of common planes consisting of a first set of electrodes which are bussed to corresponding segments in all of the symbols and segment planes consisting of a second set of electrodes which are connected to the same selected ones of the segments in individual symbols. The intersections of the first and second sets of electrodes can be thought of as similar to the intersections of an X-Y grid. The desired pattern is created by selectively energizing ones of the first and second sets of electrodes in a repeating pattern effective to energize the particular display segments in the X-Y grid necessary to produce the desired pattern.
LCDs require that the voltage applied across their elements must exceed a predetermined threshold to turn them on and that the voltage must remain below the threshold to keep them off.
In order to prevent irreversible electrochemical action from destroying the display, the voltage at each segment must be symmetrically reversed so that the average DC component of voltage across each segment is close to zero such as, for example, less than about 50 millivolts. This is accomplished using a time division multiplex system having twice the number of time divisions as there are common planes. The voltage fed to the common planes and to the segment planes within one multiplex cycle are increased or decreased at the proper times within the multiplex cycle to energize each desired segment with a voltage exceeding the threshold voltage, first with one polarity and then equally with the opposite polarity. At other times, each segment receives an alternating voltage which is less than the threshold voltage in order to maintain such segment off until the next time it is scheduled to be turned on.
The voltage threshold defining the difference between the on and off states of an LCD segment changes non-linearly with temperature. As the temperature increases, for example, the threshold voltage decreases. Thus, application of a voltage across a segment which is small enough to avoid turn-on of the segment at a low temperature is too high to permit the segment to turn off at a higher temperature. For normal indoor applications where a room temperature in the range of from about 18 to about 25 degrees C. may be experienced, the change in threshold may be small enough to be tolerated without special compensation techniques. For greater temperature ranges, temperature compensation has been employed by approximating the change in threshold voltage by a linear change in the voltages applied to the display segments.
Displays used in electric meter service are exposed to a range of environmental parameters which exceed that encountered by most such apparatus. Electric meters may be exposed to direct desert sunlight and also to arctic temperatures. I have discovered that the linear approximations to temperature compensation employed in the prior art are not satisfactory to guarantee positive control of segment energization over the entire temperature range of an electric meter.